Problem using NavigationService.GoBack()

Ive been using the Physicshelper to get a game up on Windows Phone 7. Its a great library and a joy to use. However I am having a problem.

I have physics on my MainPage.cs, then i use navigate to get to another screen (with no physics), i then use NavigationService.GoBack() and i return to the mainpage, however the physics objects are still there, but no collisions happen anymore. I have
tried using .PauseSimulation when navigating away and then enabling it again, but the same problem occurs.